Processes for reducing the concentration of NO.sub.x (a common industrial pollutant) in a NO.sub.x containing gas are well known in the art. These processes generally comprise:
(a) withdrawing the NO.sub.x containing flue gas as portion of the combustion effluent from the combustion of a carbonaceous fuel (fluidized-bed boilers are among the most efficient devices for burning carbonaceous fuels; the NO.sub.x is primarily formed from the oxidation of fuel-bound nitrogen); PA1 (b) injecting ammonia into the NO.sub.x containing flue gas in order to reduce the concentration of NO.sub.x in the flue gas (the reduction of NO.sub.x by ammonia is feasible within a narrow temperature range of approximately 1600.degree. F.-2000.degree. F. with an optimum temperature of about 1785.degree. F; at higher temperatures, the ammonia is converted to NO.sub.x while at lower temperatures, reduction of NO.sub.x by ammonia is less effective); and PA1 (c) feeding the ammonia treated flue gas to a cyclone in order to separate entrained particulate matter from the ammonia treated flue gas.
See for example European published patent application No. 176,293 by Cooper et al. U.S. Pat. No. 4,756,890 by Tang et al. teaches that the efficient mixing of the ammonia with the flue gas will result in increased NO.sub.x reductions for a given volume of injected ammonia. Tang therefore suggests that the ammonia be injected into the flue gas immediately following its entry into the cyclone's vortex region via the use of probes inserted through the top of the cyclone. There is a problem with Tang's technique, however, in that it is difficult to maintain these probes in such a hot and erosive environment for a long period of time. It is an object of the present invention to develop an ammonia injection scheme that will result in increased NO.sub.x reductions for a given volume of injected ammonia without resorting to the use of Tang's probes.